1. Field of the Invention
This invention pertains to methods of growing thin film layers. In particular, this invention pertains to growing layers of molecules using photolysis to deposit materials such as iron, silver, and compounds available through interaction of molecular compounds with ultraviolet light. Specifically, the present invention is for an apparatus using photolysis that does not require windows and can be used to produce sustained photolytic reactions.
2. Description of the Prior Art
An important technique for the control deposition of thin films of metal, semiconductors, or insulators is the decomposition of compounds containing the desired molecule. This decomposition must occur at or near a suitable substrate in a controlled atmosphere. Examples of the desired molecule are Ag, ZnS, and TiO.sub.2 for a metal, semiconductor, or insulator. The compounds containing the molecules should be relatively stable so they can be handled easily. Inorganic compounds containing the desired molecule have often been used as the deposition vehicle. It is desirable to have the compound and the vapor or gaseous phase so that it can be manipulated easily within pressure, flow, or vacuum systems.
Previous deposition techniques include sputtering, evaporation, molecular beam deposition, and chemical vapor deposition. Chemical vapor deposition involves a chemical reaction among gases to initiate decomposition. Pyrolysis is a common deposition technique in which the heated substrate decomposes the target molecule to be deposited. All the common techniques mentioned, sputtering, evaporation, molecular beam deposition, chemical vapor deposition and pyrolysis require or produce high temperatures in their application. The high temperature produces outgassing of stray or unwanted gases from the walls and components or the deposition chamber and often diffusion of chemical species from the substrate. These gases contaminate the deposited film.
The decomposition of the molecule containing compound can also be accomplished by photolysis. Photolysis uses light to initiate the decomposition. The energy absorbed from the light is sufficient to cause decomposition. Photolysis can be carried out at low temperatures. It thus offers a unique technique for depositing extremely clean films of the desired molecule on a given substrate.
Photolysis has previously been accomplished by irradiating the compound with light of sufficient energy to break apart the stable bonds attaching the molecule to the compound. This normally demands ultraviolet light and in compounds containing several bonds to the desired molecule there is a reasonable assumption that very high energy, vacuum ultraviolet light is required. Vacuum ultraviolet light will not pass through commonly available window materials, nor through the ambient atmosphere. These materials either absorb or are not transparent at the appropriate energy levels needed for ultraviolet light. Rather an evacuated path must be used to transmit the highest energy ultraviolet region which is of interest. Even if the ultraviolet light is transmitted through a special window material, deposition of the decomposed compound gas or vapor within the chamber rapidly coats the window material preventing subsequent passage of the ultraviolet light.
It is obvious that there is a need for an apparatus to carry out a method of photolysis of flowing or cyclically deposited compounds which can use highest energy ultraviolet. It is further desired that there be a photolysis apparatus which does not provide contamination by the internal production of substantial quantities of high energy ultraviolet light. Furthermore, to avoid outgassing contamination, it is desired to have on apparatus which permits the use of high energy ultraviolet within a cool, low pressure system.